1. Field of the Invention
This invention relates to micromechanical resonator devices.
2. Background Art
Recent advances in micromachining technology that yield high-Q microscale mechanical resonators may soon enable substantial size and potential cost reductions for the highly stable oscillators used in communication and timekeeper applications. In particular, IC-compatible surface-micromachined mechanical resonators from MF to VHF frequencies with Q""s in excess of 10,000 have been demonstrated in polycrystalline silicon structural materials. Prototype high-Q oscillators featuring micromechanical (or xe2x80x9cxcexcmechanicalxe2x80x9d) resonators integrated together with sustaining electronics, all in a single chip, using a planar process that combines surface-micromachining and integrated circuits, have also been demonstrated. Unfortunately, although the Q of the resonators in these oscillators is sufficient to garner respectable short-term stability, their thermal stability falls well short of the needed specifications, typically exhibiting frequency variations on the order of 1870 ppm over a 0xc2x0 C. to 85xc2x0 C. range, as shown in FIG. 1, which compares the performance of a polysilicon folded beam xcexcmechanical resonator with that of AT-cut quartz. Although techniques exist to alleviate this thermal dependence (e.g., temperature compensation circuitry, or oven control), all of them consume significant amounts of power, and thus, reduce the battery lifetime of portable devices.
In the article entitled xe2x80x9cGeometric Stress Compensation for Enhanced Thermal Stability in Micromechanical Resonatorsxe2x80x9d W.-T. Hsu et al., ULTRAS. SYMP., 1998, pp. 945-948, a geometric stress-compensation design technique is disclosed with respect to low-frequency (LF, e.g., 80 kHz) nickel folded-beam xcexcmechanical resonators that used a geometrically-tailored stress versus temperature function to cancel the thermal dependence of the material Young""s modulus, resulting in an overall lower frequency excursion over a given temperature range, and generating zero temperature coefficient TCfo, points in the process.
An object of the present invention is to provide an improved micromechanical resonator device.
In carrying out the above objects and other objects of the present invention, a temperature-compensated, micromechanical resonator device is provided. The device includes a substrate, a flexural-mode resonator having first and second ends, and a temperature-compensating support structure anchored to the substrate to support the resonator at the first and second ends above the substrate. Both the resonator and a support structure are dimensioned and positioned relative to one another so that the resonator has enhanced thermal stability.
Energy losses to the substrate may be substantially reduced to allow higher resonator device Q.
The support structure may not vibrate during vibration of the resonator.
The device may further include a drive electrode structure formed on the substrate at a position to allow electrostatic excitation of the resonator. The resonator and the drive electrode structure define a first gap therebetween which is preferably a submicron lateral capacitive gap.
The device may further include a sense electrode structure formed on the substrate at a position to sense output current based on motion of the resonator. The resonator and the sense electrode define a second gap therebetween which is preferably a submicron lateral capacitive gap.
The resonator is preferably a single resonator beam.
The support structure may include an anchor for rigidly anchoring the first end of the resonator to the substrate and a folding truss support structure for substantially decoupling the second end of the resonator from the substrate.
The resonator is preferably a lateral resonator. The support structure may include a pair of stress generating support members dimensioned relative to the resonator so that the resonator has enhanced thermal stability. The support members may be rigid against lateral motions.
The resonator may be a polysilicon resonator such as a polysilicon resonator beam.
The electrode structures may be metal such as plated metal electrodes.
The substrate may be a semiconductor substrate such as a silicon substrate.
Further in carrying out the above objects and other objects of the present invention, a micromechanical resonator device having a frequency versus temperature curve is provided. The device includes a substrate, a flexural-mode resonator having first and second ends, and a support structure separate from the resonator and anchored to the substrate to support the resonator at the first and second ends above the substrate. Both the resonator and a support structure are dimensioned and positioned relative to one another so that the frequency versus temperature curve is specifically tailored.
Such tailoring may increase temperature dependence of the resonator so that the device may be used as a temperature sensor.
Such tailoring may introduce peaks and valleys in the curve at predetermined locations.
Still further in carrying out the above objects and other objects of the present invention, a micromechanical resonator device is provided. The device includes a substrate, a flexural-mode resonator having first and second ends, and a support structure separate from the resonator and anchored to the substrate to support the resonator at the first and second ends above the substrate. Both the resonator and a support structure are dimensioned and positioned relative to one another so that the device has a substantially zero temperature coefficient temperature at which the device may be biased.
The micromechanical resonator device disclosed herein offers a method for negating the above thermal dependencies without the need for additional power consumption. In cases where power is not a large concern, the temperature-insensitive design technique of this disclosure can be combined with temperature compensating or oven-control circuits to attain thermal stabilities superior to those achievable via present-day macroscopic resonators at a given power level.
The above object and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.